If you use an epoxy resin like West Systems epoxy it won't eat the foam mold you just made...
You can buy foam blocks at most hobby stores and sculpt them to any shape you wish, wrap in fiberglass using the epoxy as resin instead of the polyester resin that gets hot and eats the foam. Then acetone will eat out the foam after the fiberglass shape is made without hurting the newly cured glass...
I've used this process to make a new rear section on my old gsxr 750 when I converted it to a solo seat, but this can be used to for just about any shape using foam, modeling clay, or anything else that's easy to shape then cover with the glass. I've also found that the glass mat works great when a lot of bulk is needed but small sections of cloth that's inter layered works best for really tight compound curves. I also made a new rear fender to replace the bulky original fender on my kz 700. On that one I used the original fender as the mold and covered the inside with a thin coat of Silicone grease, just enough to cover the surface, then put the smaller 2" x 3" squares of cloth inside the fender and built it up about 6 layers thick interlocking these layers of cloth. When it cured I just popped it out of the original fender, trimmed it so there was no part of it that could be seen below the seat level, painted it black and installed it, this way I had my own fender eliminator without cutting up the rare stock fender. It looked totally oem tho because I used the stock fender as a mold.
Pretty much, if you can think it up, you can do it, and if done right, the amount of sanding and touch up will be minimal before paint and finishing is done. You can use large sections of cloth on most surfaces then cut into small circles or squares to get into or over really tight curves. Cloth and matting can both be used on the same project as well as microspheres which are tiny glass bubbles that can be impregnated into the resin to build up areas where the glass cloth can't get into or to strengthen corners etc...